This invention relates to the dry distillation of overworn discarded rubber tires of vehicles and more particularly to a process and apparatus for continuously and smoothly dry distilling discarded tires thereby to recover gases and liquids for combustion.
As a consequence of the rapid development of motorization in recent years, there has been a tremendous increase in the rate at which overworn or worn-out rubber tires (hereinafter referred to simply as tires) of land vehicles are being discarded. Of these tires, one portion is being reutilized as retreaded tires, but most of the tires are being disposed of as refuse. Because of the shape and bulk of tires, however, they cannot be disposed of as they are as filling material for land reclamation and other purposes, and, in some instances as a stopgap lawful method of land reclamation, tires are being cut up into pieces of suitable size and then used as filling material.
Another method of disposal of such tires is the combustion thereof. On a small scale, tires are being burned outdoors in agricultural fields and orchards for the purpose of preventing frost damage. In this case, however, the gases of combustion of rubber have a characteristic bad odor, whereby tires cannot be burned in the open except in such emergency cases. The quantity of tires used in such instances is a mere 2 to 3 percent of the total quantity of tires discarded.
On the other hand a sudden interest in the utilization of tires as a combustible material is becoming apparent as a result of the rise in energy costs. This is a natural result because of the high calorific value of tires of approximately 8,000 Kcal/kg, and already most of the tires, exclusive of those being retreaded, are beginning to be used as a substitute fuel.
However, because of structural features of tires such as their characteristic shape and steel wire incorporated therein, direct combustion of these tires requires pretreatment such as cutting and an after treatment, such as the removal of steel wires from the tires after combustion. Furthermore, because of the characteristicly high surface density of rubber, the degree of contact of the rubber with air for combustion is small, whereby incomplete combustion tends to occur, and black smoke and unpleasant odors are easily given off. Combustion of tires in large quantities is difficult in actual practice without the use of large furnaces affording long combustion zones.
As another approach, it is also possible, since combustible gases and liquid fuels can be generated by dry distilling rubber at relatively low temperatures, to once extract gas and liquid fuels and to burn them in a separate combustion furnace. In this case, the steel wire and other solids are left as residue and separated, and even if the distillation furnace and the combustion furnace are installed in a spaced apart relation, continuous operation is possible by merely connecting them with transfer piping for gaseous and liquid fuels, whereby effective utilization is readily attainable.
Accordingly, numerous techniques relating to the dry distillation of tires have heretofore been disclosed and proposed. For example, Japanese Patent Publication Nos. 25874/1978 and 27752/1978 disclose apparatuses in each of which vertical preheating and dry distillation furnaces are coupled in a gas-tight manner, and, for horizontally stacking tires therein to carry out preheating and dry distillation, respectively, and means for horizontally, handling each tire for charging the tires and removing the residue must be installed. In the operation of each apparatus, preheating and dry distillation proceed as the tires being processed in the furnaces descend one at a time, being continually maintained in horizontal state, and finally the non-volatile residue is taken out.
In a furnace of this known character, support of the tires in an orderly disposition is maintained within the furnace by a tire supporting mechanism comprising a forkshaped tire receptacle provided near the bottom of the furnace and functioning as a movable grate. In the dry distillation furnace described in Japanese Patent Publication No. 27752/1978, a butterfly-shaped grate is separately provided below the support fork to function cooperatively in supporting the tire charged into the furnace and in taking out the dry distillation residue. However, after a long period of operation of these apparatuses, problems tend to arise in the taking out of the residue.
Still another proposed furnace is of a construction wherein the inner diameter of the vertical furnace is made greater than the outer diameter of the tires at the upper part of the furnace and is tapered to become smaller than the tire outer diameter at the lower part of the furnace, and this constricted lower part is caused to exhibit a grate effect to support the tires charged into the furnace. Since there are no support structures such as a grate in the interior space of this furnace, there is little possibility of substances such as residue and semi-molten material formed in the high-temperature part of the furnace being caught or adhering to parts of the furnace, and it would seem that the operation can be expected to proceed under considerably favorable conditions.
However, this furnace has a drawback in that it is difficult to take out scrap wire after completion of dry distillation and incompletely distilled residual tires frequently produced in actual operation because of the constriction at the lower part of the furnace. Particularly in the case of continuous operation, which almost always means operation over a long period, there is a continuous accumulation of residue at the bottom of the furnace, whereby removal of this residue tends to become difficult.
In the case of batch-wise operation, it is possible, upon the completion of dry distillation of each batch, to take out the residue, including scrap wire, and carbon particles adhering to various interior parts of the apparatus as described hereinafter. This work, however, is extremely troublesome and requires much time and labor. Furthermore, a batch-wise operation unavoidably entails periodic interruptions thereof, whereby a stable operation under steady conditions cannot be achieved.
In view of the above described circumstances in the state of the prior art, we have carried out a detailed analysis of the phenomena occurring within a dry distillation furnace of the instant character during operation in order to facilitate the taking out of the dry distillation residue. As a result, from a completely separate line of thinking, we have arrived at the conceptual conclusion that the cross-sectional area of the furnace at its lower part should be made the same as or greater than that of its upper part. Furthermore, we have carried out tests based on this concept, as a result of which we have succeeded in developing this invention.
More specifically, we have carried out studies on the premises:
(1) that, within a furnace reaching a high temperature, mechanisms which can become obstructions to the falling or downward movement of charged material must be eliminated as much as possible;
(2) that, since continuous operation is the general rule, mechanisms such as that for holding charged material and that for taking out residue, which are used only at the starting and stopping of operation must be dispensed with and substituted by a mechanism for continuous operation which is most easy to control and, moreover, is efficient for ordinary continuous operation; and
(3) that, for facilitating of tire charging and for uniformity of heat distribution within the furnace, the tires charged into the furnace must be stacked in random directions.
As a result, we have found that, during steady operation, the charged tires, the residue of these tires, and the like, while combining naturally to form suitable gas passages within the furnace, brake and retard the charged material in the upper part of the furnace, whereby not only is a grate unnecessary, but since the combined mechanism progressively varies, it is useful in the stabilization of the furnace condition. Moreover, since the dry distillation residue is predominantly steel wire, it is extremely bulky, and if there are some obstructions within the furnace, this steel wire is readily caught thereby, whereby the taking out of the residue is hindered.
Therefore, it was verified that expanding the furnace cross-sectional area in the downward direction is desirable for facilitating the work of taking out the residue, that at least there is absolutely no necessity of constricting the lower part of the furnace relative to its upper part, and that, if the lower diameter is smaller than the upper diameter, trouble occurs frequently during the work of taking out the residue.
It was confirmed further that when the inner diameter of the furnace is made constant or is downwardly expanded, bridging blockage or a so-called log-jamming effect of the charged tires as they naturally fall or move downward is prevented, and that, in addition, the charged tires are automatically restacked as they sink while they are dry distilled and burned, whereby uniform reaction is facilitated, and the solid materials exhibit an effect of forming their own grate. These effects cannot be obtained in systems wherein tires are charged horizontally one at a time.
On the basis of the above described findings and conclusion, we have developed a process and furnace for continuously dry distilling tires over long periods of time. However, still another problem must be solved before this continuous and long-period dry distillation of tires can be profitably practiced. That is, one hindrance to the continuous and long-period distillation of tires has heretofore been the presence of a great quantity of dust particles, comprising principally carbon particles, in the gases generated in and discharged from the tire dry distillation process step.
More specifically, oils produced in the dry distillation adhere to these dust particles (hereinafter referred to as carbon) to form sticky carbon, which adheres to the inner wall surfaces of the apparatus and parts such as complicated bent parts and gas passages and, upon accumulating, gives rise to various difficulties such as clogging of passages. Accordingly, we have developed an apparatus for dry distilling tires which can be operated continuously and over long periods under stable conditions within the dry distillation furnace, and in which removal of adhering carbon can be accomplished without stopping the operation of the apparatus.